Method of making voltage-dependent resistors

ABSTRACT

Voltage-dependent resistors comprising a sintered, homogeneous body of zinc oxide, and a small distribution of C-values, are made by first calcining the mixture of zinc oxide, as a main constituent, and bismuth oxide, as a first additive(s), and then by adding further additive and sintering the mixture.

United States Patent 11 1 Iga et al.

" ;;[;;-;1" '1 3,903,226 1 1 Sept. 2, 1975 METHOD OF MAKING VOLTAGE-DEPENDENT RESISTORS [75] Inventors: Atsushi Iga, Takatsuki; Osamu Makino, Hirakata; Michio Matsuoka. Ibaragi; Takeshi Masuyama, Takatsuki, all of Japan [73] Assignee: Matsushita Electric Industrial Co.,

Ltd., Japan [22] Filed: Dec. 16,1974

[21] Appl. N0.: 533.090

3,037,942 6/1962 lngold ct ul 252/520 X 3.617.592 11 /1971 Arr-6666.; 106/395 x 3.805.114 4/1914 Mntsuolcn 01.111. 252/5 18 X FoREiqN Oil APPLICATIONS 2.023.507 s' '19'7o1j1=l-dn j OTHEQElfUiiLICAiiONS Chem. Abstracts, 7 1119711 132] 12 Gupta, T. K.; lnhibit ionp f Grziin Grov vth in ZnOXJQ Am. Cer. Soc/454' 1-,97 11), pp. 41-3 414;

Primary liyunziner-Hlen M. McCzirthy Armrnqv. Agent, 0r: .Firm*Wendroth Lind & Ponack 57 ABSTRACT Vbltage-dependent resistors comprising a sintered. 110- mogeneous body ofizinc oxide/ and asmall distribu;

tion of C-values, arernad e 'by first calciningthe mixture of zinc oxide, as a main constituent. bismuth oxide as n first additivds); and then-by adding further additive and sintering the mixture. 1

6 Claims, N0 Drawings Mme ,oF .MA gino QCLTAGEZLDEPE m: REssT'ojRs I u is L w I This invention relates to a method ma age-dependent resistors haying'nonolimic resistance due to the bulk thereof and I more particulzirly to a .are two reasons 'for' such distribution of the C-valuei' One is due to the distribution of lapped thickness of the 1 method" of makingvaristors comprising zinc oxideand bismuth oxide with less distributedvaristor .voltfage.

Various voltage-dependent. resistors suchas silicon carbide varistors. selenium rc ctificrs and germanium or silicon p-n junction dio des have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltagedependent resistor are expressed by the relation:

where V is the voltage across the resistor. l the current flowing through the resistor. C is.a constant corresponding to the voltage at a given'current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

. litg l 1 1,

" 7; log l. ./I',)

where V and V. are the voltages at a given currents! and .1 respectively. The desired value of C depends upon the kind of application to whichfthe resistor is to be. put. it is ordinarily desirable that the value of n be as large as possible since this exponent determines the gdegree to..which the resistorsdepartfrom ohmic characteristics.

Certain prior art relates to voltage-depenent resistors comprisinga zincoxide sintered body having electrodesapplied to the opposite surfaces thereof. for ex ampleas seen in US. Pat. No. 3.66 3.458. US. Pat No. 3 ,663.45 8 discloses voltage-dependent resistors having voltage-nonlinearity due to the bulk thereof and being characterized by a high n-value. That is. there is provided a voltagcdepcndent resistor comprising a sintered body in, a composition consisting essentially of 80.010199!) mole of zinc oxide. 0.05 to 10.0 mole percent of bismuth oxide and 0.05 to 10.0 rnole per- 2 .ues. aniito achieve this. q

should well controlled.

a owever. in the con'ventional voltage-dependent r'e- ..sistors; coniprisinga zinc oxide sintered body: there is a defect in.that the C-yal'ueis notconstant and it is impossible to avoid some distribution of C-valucs. There sintered body and the'other'is due to distribution of each C-value itself per unit thickness. The latter is considered due to.inhomogeneity in the. sintered body. Since. the thickness of the lapped bodycan be con-. trolled within a rather high accuracy suchas ilf/r. a

wide distribution in the C value is mainly due to the inhomogeneity in the sintered body. In order to reduce such the distribution in the C-value. there have been performed some trials. for example. to change the conditions of manufacturing the sintered body such as the time of pulverising or mixingealcining temperature.

sintering temperature or time. and so on. As the C- value. per unit thickness changes with each produced tribution of the grain size in the sintered body. a current focuses on the grains of a large grain size rather than small grains of high resistance and so the distribution of the value. i.c. the inhomogeneity-in the sin- ,tcred'body. causes degradation of the voltage-dependentcharacteristics of the resultant resistors.

Therefore. an object of the present invention is to provide an improved voltage-dependent resistor having stable characteristics. V A A furtherobject of the present invention is to provide a voltage-dependent resistor comprising a sintered body of zinc oxide and having a stable C-value over a wide current range.

A further object of the invention is to provide a novel J and improved method of making a voltage-dependent resistor comprising a homogeneous sintered body of zinc oxide. and thereby being characterized by the stacent. in total. ,of at least one member selected fromthc group consisting of cobalt oxide. manganese oxide. in- .dium oxide, antimony oxide, titanium oxide. boron oxide. aluminium oxide.'tin oxide. bariumo xide. nickel oxide molybdenum oxide.-tantalum oxide. iron oxide.

.and chromium oxide and ohmic electrodes applied to distance results in a lowehChgrlue. The bulk type voltage-dependent resistor thusobtained has a high n-value as well as stability against eurrentwand canbe widely used for stabili/lationof voltage or surge teristics be stable and in the ordered range of the val- "ble characteristicsf These objects are achieved by the method of the prjesent'invention. which comprises the steps of preparing a mixture of zinc oxide. as a main constituent. and

'();l and 5.0 mole percent of bismuth oxide (Big03)', calcining the thus prepared mixture of zinc oxide (ZnO) and 'bismuth oxide (SL 0 at a temperature between 650 and 950C; adding to the thus calcined mixture 21 further additive component comprising at least one member selected from the group consisting of 0.] to 5.0 mole percent of cobalt oxide (C00) and ().'l to 5.0 mole percent of manganese oxide (M pulverizing the thus made mixture having a further additive component; sintering the thus pulverized mixture so as to obtain a sinteredgbody; and applying electrodes to opposite surfaces of said sintered body.

The sintered body can beprepared by a per se well known ceramic technique. The starting materials in the compositions of zinc oxide and bismuth oxide are mixed in a wet mill so as to produce homogeneous mixtures The rnixturcsare dried and calcined in air. and after calcining. further additive components are added to the calcined mixture; and mixed therewith. After that. the resultant mixtures are pulverized in a wet mill so as to produce homogeneous mixtures. These mixualityof the produced device ity and stability The preferred amount of bismuth oxide to be added to zinc oxide is ().l to 5.0 mole 71 to provide the effect of the process of thisinvention. Although in the Example represented hereinafter. all of the bismuth oxide is added to 2ine oxide and the mixture is calcined. it is alsopossible to added a part of the total amount.which is defined as ().l to 5.0'rnole of bismuth oxide to zinc oxide as the starting mixture i.e. at the time of calcining and to add the remainder to the calcined mixture. to-

' gether with the further additives. i.e. cobalt oxide and 'manganese oxide. at the time of pulverising. In the latter case. the amount of bismuth oxide to be first added,

I to Zinc oxide for calcining should be at least more than 0:05 mine /1. The effect of providing the homogeneous sintered body in the latter case is also remarkably similar to the fomier case wherein all of the bisniuth oxide is added at once to zinc .oxide prior to calcining. Further. the other bismuth component may be employed instead of bismuth oxide. if it is converted to the oxide at a high temperature during calcining and s'ntering.

With respect to the aforesaid sintered body of zinc oxide having combined addition of bismuth oxide and at least one metal oxide selected from the group consisting ofcobalt oxide and manganese oxide. thcpreferable firing temperature ranges from l()()() to l 45()C. For the temperature of calcining the starting mixture of zinc'oxide and bismuth oxide. the preferable range is 650C to 9509C. The calcining temperature lowcrthan 650C is not practical because it results in too slow a reaction. and a calcining temperature higher than 950C is undesirable because it results in ovcrsintering and therefore difficulty of pulverization.

As shown in the Examples represented hereinafter. to immprove the electric characteristics of the resultant resistors. it is advantageous to add further additive components. besides cobalt oxide and manganese oxide. to the pulverized mixture of the starting materials.

i.e. zinc oxide and bismuth oxide. such components as Sb. Ti. Be. Sn. Cr. Si. Ni. Mg. Ba. B. etc. Further. there,

additive components are not limited to the oxide. and

there may be employed fluorides and carbonates.

faces thereof. with" aforesaid ohmic electrodes in any conventionally available and suitable method.

Lead wires can be attached to the silver electrodes in a per se conventional manner by using conventional" solder having a low melting point. t 1

The C-values of the voltage-dependent resistors made according tothe method of the invention have a small distribution and the resistors have ahigh stability with respect to temperature. which'is confirmed. for

- example in the load life test carried out.at-7()C at a rating power for 500 hours. The n-values and C-valucs of the resultant resistor do not change too much even Y the resultant voltage-dependent resistors be embedded in a humidity proof resin such as an epoxy or phenol resin ina per se well known manner.

An embodiment of the invention is described in the following Examples. in which. to define the distribution of the 1C-values. standard deviation is used. That is.

when the samples of n pieces of voltage-dependent resistors have the C-values of C C J C 3 l C,,. respec- ='tively. the central value C,, of these C-values and the equations;

Since the C-value is a voltage between the electrodes of the voltage-dependent resistor at a' given current as described hcreinbefore.'o,. is also represented in a units of voltage. Practically. it is'conven'ient to define the C value with a current oflmA.

EXAMPLE I The starting materials. zinc oxide (ZnO) and bismuth oxide (Bi- 0 listed in Table l. were mixed in a wet mill furnace-cooled to room temperature. Further additives components listed iiiTable l were added to the caleined mixture and'thc mixture was pulverized in a wet mill for 20 hours. The pulverized mixed powder'was dried and pressed in a mold into a disc of l7.5 mm diameterand 2.5mm thickness under a pressure of 340 kg/m The pressed body was sintered in' air for 1 hour atl3 50 C and then furn'ace-cooledto room tempera ture. The sintered disc was lapped to the thickness of 1 mm at the opposite surfaces thereof by a silicon carbide abrasive with a particle'of 600 mesh. The opposite surfaces of sintered disc were provided with an evaporated fil'm'of indium metal.

The electrical characteristics of the resultant resistor were measured for I 100 resistor pieces and. the results 0f the measurements are shown in Table 1. Itwill be readily undcrstood'that the mi-values'have a tendency to become nearly equal to each other independently of the C-valuc. When the voltage-dependent resistors. which have the same size as that of this Example, were made by the usual process. in which any kind of calcining is'not performed. the zn-values of the resultant lOO resistors were all larger than 5. In case of calcining the mixture of zinc oxide and all of the additives. i.e. bismuth oxide. cobalt oxide and manganese oxide. at a temperature between 650C and 950C. the a,.-value ranged from 4.() to 5.0. for 100 pieces, showing little reduction of inhomogeneity and distribution of the C- value. Also. both the C-values and then-values of the 1 resultant voltage-dependent resistors were increased a little. respectively. On the contrary. the m-values of the voltage-dependent resistors made bythe method of the present invention were from 2.3 to 4.0. as shown in 'lable l. wit h the.h igher n-values and .C-values. The preferred cal' 'n rig-.terriperature for the mixture of zinc oxide (ZnO) withbismuth oxide (Bi O ranges from 750fC to 850C as understood from Table l.

liABl.

Stz1r1.ing Mun-rink (mule l'lucllic-('l1:1ra|Llcri$Iius (ulciniflg 7' .1 1'Rc5ull1111t Resistors" 1 Temperature 'First 1 Further A1l1li1i\ e i 1 "-1 i Z110 Additive ('ninpunent 1:11 giwn i n B1203 (.110 M1102. -lm.-\1 1.l--In1A (1511 1",11 1.5 If '4 3.3 l3.2 11511 111.11 5.11 5.11 15 3. 1 7.1 6511 94.) (Ll 5.1) (12 3.7 (1.8 11511 14. 1 5.11 11.1 73 3.4 1.11 7511 1 1.5 11.1 11.1 52 2.4 111.1 7511 1 1.11 11.5 11.5 111 2.4 15.1 7511 17111 1.5 1.5 7. -2.11 14.5 7511. 111.11 5.11 5.11 111 2.5 7.5 7511 14. 1 11.1 5.11 11.1 3.7 11.7 7511 14. 1 5.11 1 11.1 75 2.11 5.5 11511 1 1.5 11.1 11.1 55 1 2.11 1.7 5511 1 1.11 11.5 11. 112 2.5 17.5 .5511 17.11 1.5 1.5 511 2.5 14.11 1 11511 111.11 5.11 5. 11 1111 3.11 5.4 8511 14. 11.5 5.11 115 2. 1 5.7 15511 14. 1 .11 11.1 .51 2.7 1.3 1511 1 1 1.5 11.1 11.1 55 3.7 1.2 .9511 1 1.11 11.5 11.5 113 3.11 17.11 1511 17.11 1.5 11.5 53 3.3 14.11 1511 111.11 5.11 5.11 1113 3.7 7.. 1511 14. 1 11.5 5.11 711 3.11 7. 1 1511 14. 11 5.11 11.1 s?- 3.4 5.1 11511 1 1.5 11.1 11.1 42 3.3 7.1 11511 1 1.11 11.5 11.5 44 3.1. 15.7 11511 17.11 1.5 1.5 55 3.2 11. 1 11511 111.11 5.11 5.11 7.1 3.11 7.4 11511 14. 1 11.1 5.11 4 1 3.1 5.3- (1511 1 14. 1 5.11 11.1 11.5 3.4 11. 1 7511 1 1 1.5 11.1 11.1 44 2.4 7.3 7511 1 1.11 11.5 11.5 -15 2.3 17.2 7511 17.11 1.5 1.5 57 2.3 1. .2, 7511 111.11 5.11 5.11 7 1 2. 1 7.3 7511 94.) (1.1 5.1) 47 2.5 ).l 7511 1 14. 1 5.11 11.1 711 2.4 7.5 K51) 19.11 1) l 1).| 43 2.3 7.8 11511 1 1.11 11.5. 1 Y 11.5 411 2.4 17.1 .851) 17.1) l.5 L5 54 2.4 l2.5 21511 111.11 5.11 5.11 7 2.5 7.11 8511 14. 1 11.1 5.11 4 1 2.11 1.4 4451) 94.). 5.11 11.l 72 2.5 7. 1511 1 1.5 11.1 11.1 47 3. 1 5.2 1511 1 1.11 11.5 11.5 421 3.1 111.5 1511 a 17.11 1.5 1.5 5 1 3.11 12.11 1511 111.11 5.11 5.11 7 1 4.11 7 3 1511 1 14. 1 11.1 5.11 53 1. 1 .52 1 1511 14. 1 5.11 11.1 73 3.7 711 11511 1 1.7 11.1 11.1 11.1 3. 1 1 11.7 11511 114 .11 11.5 11.5 11.5 4. 1 5.4 111.1 11511 11 1. 1 11.1 5.11 5.11 4 1 3.4 7.1 11511 1 1.3 11.5 11.1 11.1 47 3.11 1.15 6511 98.5 11.5 11.5 (1. 5] 3.2 15.2 11511 11 1.5 11.5 5.11 5.11 111 3.5 12.11 7511 111.3 1.5 11.1 11.1 (111 2.4 11.2 7511 1 17.5 1.5 11.5 11.5 1111 2.4 15. 1 7511 81525 1.5 5.1) 5.1) H4 2.7 7.2 7511 94.8 5.11 11.1 11.1 71 2.3 11. 1 7511 14.11 5.11 11.5 11.5 7 1 2.3 11.2 7511 $5.11 5.11 5.11 511 H14 2.7 1 11.11 14511 1 1.7 11.1 11.1 11.1 47 2.7 11.4 8511 98.9 1).l 11.5 (1.5 52 2.5 111.5 11511 11 1. 1 11.1 5.11 5.11 2.14 (1.8 11511 1 1.3 11.5 11.1 11.1 4 1 2.11 111.11 11511 111.5 11.5 11.5 11.5 54 2.4 17.11 11511 11 1.5 11.5 5.11 1 5.11 113 2.11 11.2 1511 111.3 1.5 11 1 11.1 111 3.4 12.4 951) 97.5 L5 (1.5 1.5 61K 3.l 18.2 9511 88.5 LS 5.11 5.11 3.) 12.1 1511 14.11 5.11 11.1 11.1 73 3.4 5.5 1511 14.11 5.11. 11.5 11.5 141 1 1 13.3 1511 145.11 5.11 5.11 5.11 x 1 1 a 12.5

Although not shown in Table 1. similar results with respect to small tz1ndz1rd deviation of distribution of the C-values with the high C-values and n-values were the calcined mixture. the remainder of the first addiobtained when the starting mixture of zinc oxide and a tivc. and 21 further additive component. Le. C and/0r part of the defined amount of the first udditive. i.e. his- MnO- were sintered. Similar results were also 11bmuth oxide. were calcined and after pulverization of l served in the following Examples 2 to 5.

EXAMPLE 2 The starting materials. zinc oxide (ZnO) and bismuth oxide (Bi- O listed in Table 2. were-mixed 'in'awet 8 EXAMPLE 3 The starting materials, zinc oxide (21101 and bismuth oxide (B1 listed in Table 3. were mixedin a wet mill for 5 hours. The mixture thus obtained was dried mill for 5 hours. The mixture thus obtained was dried 5 and calcined in air' for .1 hour at 750C and furnace and calcined in air for 1 hour at 750C and furnacecooled to room temperature. Further additive compo cooled to room temperature. Further additive compo- .nents listed in Table 3 'wcreadded to the calcined mixnent listed in Table 2 were added tothejcaIcined mixture and thus added mixture was pulverized in a wet ture andthus added mixture was pulverized in a wet mill for hours. Then. the pulverized mixture was mill for 20 hours. Then. the pulverized mixture was dried. pressed. sintered and lapped in the same manner dried. pressed. sintered and lapped in thersame manner as that in Example 1. and the electrodes were applied as in Example 1. and the electrodes were applied to the to the resultant bodies. similarly to Example 1 The resultant bodies. similarly to Example 1. The electric electric characteristics of the resultant resistors are characteristics of the resultant resistors are shown in shown in Table 3, in which the a,.-value is calculated Table 2. in which the 0',.-value is calculated for 100 rc- 15 for 100 resistor pieces. While the (r,.-value of resistors sistor pieces. While the (TU-VZIIUC of resistors having the having the same composition as that of Table 3 and same composition as that of Table 2 and being made by being made by usual process ranges from 2.0 to 3.5. the usual process ranges from 2.5 to 4.0. the (r,.-value of the ray-value for the invention is remarkably reduced to 1. l C-values of the resistors made by the method of the into 1.6. as shown in Table 3.

' Table 3 Starting Materials (mole First Electric Characteristics of Resultant Resistors C- n Additive Further Additive Component (at given current of 01. 0.1-lmA Z110 Bi O: (o0 MnO S11 0 SnO- Cr O SiO. NiO MgO BaO B 0 lmA) 2.2. '1 w 1 vention 1s remarkably reduced to l 7 to t 5 sho 1 LE 4 Q in Table 2.

The starting materials. zinc oxide (ZnO) and bismuth oxide (B1203), listed in Table 4, were mixed in a wet Table 2 Electric Characteristics Starting Materials (mole '2? of Resultant Resistors First C n Additive Further Additive component (at given 0,. Z110 Bi O ("o0 MnO Sh. ,O TiO. BeO current of 0.1-1mA lmA) 98.98 0.5 0.5 0.02 42 2.2 25 98.98 0.5 0.5 0.02 2.0 26' 9 8.48 0.5 0.5 0.5 0.02 51 1.8 26 98.0 0.5 0.5 1.0 2.1 29 98.0 0.5 0.5 1.0 77 2.1 31 97.5 .5 0. 0.5 1.0 77 2.0 34 98.5 0.5 03 0.5 21 1.8 20 98.5 0.5 0.5 0.5 28 1.8 22 f 98.0 0.5 0.5 0.5 0.5 25 1.8 23 97.5 0.5 0.5 1.5 32 1.7 22 97.5 0.5 0.5 1.5 39 1.8 23 97.0 0.5 0.5. 0.5 1.5 39 1.x 2 24 93.0 0.5 0.5 0.0 is 1.7 19'. 93 0.5 0.5 6.0 21 1s ,20 1 I 92.5 0.5 0.5 0.5 6.0 20 1.7

. 19 7 mill for hours. The mixture thus obtainedavas dried andcalcined in air for 1 hour at 750C and furnaccc00led1t1 room temperature. Further additive components lis ed i117r11111e 4were added to the calcined mix ture and thus added mixture was pulverized in a wet mill for 20 hours. Thpflthe pulverized mixture was dried. pressed. sinteretliahd lapped in the1same manner 1115111111915 1. arid the electrodes were applied to the resultantbodie." similarly to Example l. The electric characteristics of the resultant resisttws are shown in-Tah1e 4. in which the 1r,.-value is calculated for 100 resistor pieces. While the 1r,.-value of resistors 10 mill for 5 hours. The mixture thus obtained was dried and calcined in air for 1 hour at 750C and furnacecoolcd 10111110111 temperature. Further additive components listed ip l 'ahle5 were added to the calcined mixture and thus added mixture was pulverized in wet mill for hours. Then. the pulverized mixture was dried. pres ed. SillICfQLlllfld lapped in the 511111011111111181111811111! in Exiunplel. and the electrodes were applied to the resultant-bodies. similarly to Example 1. The electric characteristics of the resultant resistors are shown in Table 5. in which the 1r,.-value is calculated for 100 resistor pieces. While the 1r,.-value of resistors having the same composition as that of Table 5 andb eing made by usual process ranges from 2.0 i' -value for theinv ention 1. remarkably rducbd 111] to 1.5. as shown 1111111 e i i i i Eleetrie Characteristics of Resultant Resistors Furthchkdditive component 1 K 1 C 1 1 i i 11 (at given current 1r,. 0.l lmA '1 Nio MgO 11.11) 11.1 0 (110 (1111 111 lmAl 1 11.5' 11.5 4s 1. 1.5 2x1 111. 11.5 511 1.5 31 11.5 11.5 51 1.11 311 1135" 11.5 :4 1.3 :3 511.51 11. 311 1.5 :5 .11.5. 11.5 2 7 1.5 25 1.5 11. r 25 1.4 :1 11.5 1 11.5 "s 14 :3 ..11.5 11.5 as 1.5 2: 11.5 11.5 2 1 1.5 211 (1.5 11.5 311 1.5 1. 4 11.5- 11.5 11.5 1 3s 1.3 .34 11.5 11.5 11.5 44 1.5 3-1 11.5 11.1 11.115 :1 1.2 27 11.5 11.1 11.5 2: 1.2 28

-EXAMB1 a AS starting materials, 2111c oxide (ZnO) and bismuth "oxide (BigO hp listed iaTable 5. were mixed in a wet Table 5 1 1 11Starting Materials(mole.'

Electric Characteristics of Resultant Resistors First C a Additive Further Additive component (at given 11 current of 11;. ZnO B1 0 CoO M110 BeO SnO Cr. SiO NiO MgO [3110 B 0 (d0 a0 lmA 1 0. l- 1 mA 911.5 11.5 11.5 2.11 11.5 51 1.4 2-1 96.5 0.5 0.5 2.0 0.5 5(1 1.4 25 96.5 0.5 0.5 0.5 2.0 0.5 54 1.5 26 91.95 0.5 0.5 6.0 0.05 11 1.5 16 92.95 11.5 11.5 11.11 11.115 3.1 is 92.45 11.5 11.5 11.5 11.11 11.115 33 L5 96.5 11.5 0.5 2.0 0.5 55 4 3. 96.5 0.5 0.5 1.0 0.5 5) 1.4 27 96.0 0.5 0.5 0.5 2.0 0.5 53 1.4 25 92.5 0.5 0.5 6.0 0.5 32 1.4 17 92.5 0.5 0.5 (1,0 0.5 315' 1.4 I 92.0 0.5 0.5 0.5 (1.0 0.5 38 1.5 19 96.5 0.5 0.5 0.5 2.0 0.5 31 1.4 25 961.5 0.5 0.5 0.5 2.0 0.5 33 1.4 25 90.0 0.5 0.5 0.5 2.0 0.5 33 1.3 :6 92.5 0.5 0.5 0.5 (1.0 0.5 18 1.5 15 92.5 0.5 0.5 0.5 (1.0 0.5 1) 1.4 15 92.0 0.5 (1.5 0.5 (1.0 0.5 20 1.5 16 95.9 0.5 0.5 0.5 2.0 0.1 0.5 35 1.3 3H 11.5 11.5 11.5 2.11 11.1 11.5 24 1.: 37

. Table 5-continued Starting Materials imolc"i l-Ilectrie Characteristics of Resultant Resistors I First- 1 Additive Further Additive component g (at gi en n I v I current of m. Z110 B1 0 (0( MnO- BeO $110;- 0,; "SiO NiO MgO -BaO B 0 (do (110" lmA) (Ll-lmA as 05 0.5 v 11.5 2.0 '011 0.5 I 36 1.3' 324 2.4 0.5 0.5 11.5 (.0 0.1 1:4 1.4 21 92.4 0.5 0.5 0.5 (1.0 0.] 0. I l b 1.3 24 92.2 0.5 0.5 0.5 0.0 0.1 0.1 0.1 Y 1.2

What is claimed is: r I

l. A method of making a voltage-dependent resistor comprising zinc oxide. as a main constituent. and 0. l to 5.0 mole percent of bismuth oxide (Bi; O.-;). as a first additive. said method comprising the steps of preparing a mixture comprising zincoxide and the balance at least more than 0.05 mole percent of bismuth oxide. calcining the thus prepared mixture of zinc oxide (ZnO) and bismuth oxide (BL-I03) at "a temperature between 650 and 950C. adding to the thus calcined mixture a further additive comprising at least one member selected ther additives comprise at least one member selected from the group consisting of 0.1 to 5.0 mole percent of cobalt oxide (C00) and 0.] to 5.0 mole percent of manganese oxide (MnO and one member selected from the group consisting of 0.0] to 5.0 mole percent of antimony oxide (Sb- ,O to 5.0 mole percent of titanium oxide (TiO. and 1.0 to l0.0 mole percent of beryllium oxide (BeO).

3. A method as claimed in claim 1. wherein said further additive comprises at least one memeber selected from the group consisting of 0.] to 5.0 mole percent of cobalt oxide (C00) and (H to 5.0 mole percent of manganese oxide (M110 0.0l to 5.0 mole percent of antimony oxide (Sb. ,O and at least one member selected from the group of. 0.02 to 3.0 mole percent of tin oxide (SnO 0.02 to 3.0 mole percent of chromium'oxide (Cr- 0 0.l to 10.0 mole percent of silimole percent of cadmium oxide (CdO).

con dioxide (SiOQL-OJ to 5.0 'mole percent of nickel oxidetNiO). 0.] to 5.0 mole percent of mangesium oxide (MgO). 0;02 to 5.0 mole percent of barium oxide (BaO) and 0.02 to 5.0 mole percent of boron oxide (B2 .'l)- Q 4. A method claimed in claim 1., wherein said furtheradditive comprises at least one member selected from the group consisting of 0'. l to 5 .0 mole percent of cobalt oxide'(CoO) and 0.1 to 5.0 mole percent of manganese oxide (MnO- 0.l to 5.0 mole percent of titanium oxide (TiOl and at least one member selected from the group of. 0.02 to 3.0 mole percent oftin oxide (SnO. 0.02 to 3.0 mole percent of chromium oxide ((3 0 0. l to 10.0 mole percent of silicon oxide (SiO 0.l to 5.0 mole percent of nickel oxide (NiO):, 0.1 to 5.0 mole percent of magnesium oxide (MgO). 0.02 to 5.0 mole percent of barium oxide (BaO). 0.02 to 5.0 mole percent of boron oxide (B 0 0.1 to' S.0 mole percent of calcium oxide (C110) and ().l to 5.0

5. A method as claimed in claim 1, wherein said further additive comprises at least one member selected from the group consisting of O. l to 5.0 mole percent of cobalt oxide (C00) and 0.1 to 5.0 mole percent of manganese oxide (MnO 1.0 to l().() mole percent of beryllium oxide (BeO); and at least one member selected from the group of 0.02to 3.0 mole percent of tin oxide (SnO 0.02 to 3.0 mole percent of chromium oxide (Cr- 0 0.1 to 10.0 mole percent of silicon oxide (S0,). 0.l to 5.0 mole percent of nickel oxide (NiO), 0.1 to 5.0 mole percent of magnesium oxide (MgO), 0.02 to 5.0 mole percent of barium oxide (BaO), 0.02 to 5.0 mole percentof boron oxide (B 0 0.1 to 5.0

molepercent ofv calcium oxide (CaO) and 0.1 to 

1. A METHOD OF MAKING A VOLTAGE-DEPENDENT RESISTORR COMPRISING ZINC OXIDE, AS A MAIN CONSTITUENT, AND 0.1 TO 5.0 MOLE PERCENT OF BISMUTH OXIDE (BI2O3), AS A FIRST ADDITIVE, SAID METHOD COMPRISING THE STEPS OF PREPARING A MIXTURE COMPRISING ZINC OXIDE AND THE BALANCE AT LEAST MORE THAN 0.05 MOLE PERCENT OF BISMUTH OXIDE, CALCINING THE THUS PREPARED MIXTURE OF ZINC OXIDE (ZNO) AND BISMUTH OXIDE (BI2O3) AT A TEMPERATURE BETWEEN 650* AND 950*C, ADDING TO THE THUS CALCINED MIXTURE A FURTHER ADDITIVE COMPRISING AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF 0.1 TO 5.0 MOLE PERCENT OF COBALT OXIDE (COO) AND 0.1 TO 5.0 MOLE PERCENT OF MANGANESE OXIDE (MNO), AND THE REMAINDER OF THE FIRST ADDITIVE BISMUTH OXIDE (BI2O3) WHEN THE TOTAL AMOUNT OF 0.1 TO 5.0 MOLE PERCENT THEREOF IS NOT ADDED IN THE PREVIOUS STEP OF CALCINING, PULVERIZING THE THUS MIXTURE CONTAINING THE FURTHER ADDITIVE, SINTERING THE THUS PULVERIZED MIXTURE SO AS TO OBTAIN A SINTERED BODY, AND APPLYING ELECTRODES TO OPPOSITE SURFACES OF SAID SINTERED BODY.
 2. A method as claimed in claim 1, wherein said further additives comprise at least one member selected from the group consisting of 0.1 to 5.0 mole percent of cobalt oxide (CoO) and 0.1 to 5.0 mole percent of manganese oxide (MnO2) and one member selected from the group consisting of 0.01 to 5.0 mole percent of antimony oxide (Sb2O3), 0.1 to 5.0 mole percent of titanium oxide (TiO2), and 1.0 to 10.0 mole percent of beryllium oxide (BeO).
 3. A method as claimed in claim 1, wherein said further additive comprises at least one memeber selected from the group consisting of 0.1 to 5.0 mole percent of cobalt oxide (CoO) and 0.1 to 5.0 mole percent of manganese oxide (MnO2); 0.01 to 5.0 mole percent of antimony oxide (Sb2O3); and at least one member selected from the group of, 0.02 to 3.0 mole percent of tin oxide (SnO2), 0.02 to 3.0 mole percent of chromium oxide (Cr2O3), 0.1 to 10.0 mole percent of silicon dioxide (SiO2), 0.1 to 5.0 mole percent of nickel oxide (NiO), 0.1 to 5.0 mole percent of mangesium oxide (MgO), 0.02 to 5.0 mole percent of barium oxide (BaO) and 0.02 to 5.0 mole percent of boron oxide (B2O3).
 4. A method as claimed in claim 1, wherein said further additive comprises at least one member selected from the group consisting of 0.1 to 5.0 mole percent of cobalt oxide (CoO) and 0.1 to 5.0 mole percent of manganese oxide (MnO2); 0.1 to 5.0 mole percent of titanium oxide (TiO2); and at least one member selected from the group of 0.02 to 3.0 mole percent of tin oxide (SnO2), 0.02 to 3.0 mole percent of chromium oxide (Cr2O3), 0.1 to 10.0 mole percent of silicon oxide (SiO2), 0.1 to 5.0 mole percent of nickel oxide (NiO), 0.1 to 5.0 mole percent of magnesium oxide (MgO), 0.02 to 5.0 mole percent of barium oxide (BaO), 0.02 to 5.0 mole percent of boron oxide (B2O3), 0.1 to 5.0 mole percent of calcium oxide (CaO) and 0.1 to 5.0 mole percent of cadmium oxide (CdO).
 5. A method as claimed in claim 1, wherein said further additive comprises at least one member selected from the group consisting of 0.1 to 5.0 mole percent of cobalt oxide (CoO) and 0.1 to 5.0 mole percent of manganese oxide (MnO2); 1.0 to 10.0 mole percent of beryllium oxide (BeO); and at least one member selected from the group of 0.02 to 3.0 mole percent of tin oxide (SnO2), 0.02 to 3.0 mole percent of chromium oxide (Cr2O3), 0.1 to 10.0 mole percent of silicon oxide (SiO2), 0.1 to 5.0 mole percent of nickel oxide (NiO), 0.1 to 5.0 mole percent of magnesium oxide (MgO), 0.02 to 5.0 mole percent of barium oxide (BaO), 0.02 to 5.0 mole percent of boron oxide (B2O3), 0.1 to 5.0 mole percent of calcium oxide (CaO) and 0.1 to 5.0 mole percent of cadmium oxide (CdO).
 6. A method as claimed in claim 1, wherein said step of calcining is performed at a temperature between 750* and 850*C. 